curved indentation surface
Note: I've used method from here: How to save curve shape of cylinder with
  such deform? with Shrinkwrap modifier to place vertices in oval
  shape without distorting overall mesh shape.


I think topology from below image is probably the cleanest possible. It allows to add Edge Loops to control hardness to the edges of indentation or you can use Mean Crease there.

I've made indentation by pushing vertices with Proportional Editing enabled.